TR6 PDWA valve

[mhossack]

Hi my question is, on a 1975 US TR6 can you remove a brake PDWA valve piston on the car.

My PDWA valve is leaking through the switch so I need to stop this.

My options.

1: do nothing other than cap the switch with a 3/8 UNF bolt and copper washer, my worry about doing this, if one side of the piston rubbers fail completely would this stop the equal pressure keeping the piston central and possible only having either the front or rear brake circuit working, I do not know the travel of the piston left to right.

2: remove the piston and re rubber the piston, this would be the safest option keeping the braking system to US spec.

I have read about the problems removing the piston, can this be done on the car, I have not yet lost my brake pedal but it must be drawing in air.

I am sure someone must have tried this, if you cap the switch and remove the piston bore cap and press the brake pedal, will this eject the piston.

3: if the piston is removed has anyone not put the piston back in and run the car with one brake circuit like UK cars, I am worried that the 2 circuits could have different pressures from the brake master cylinder and cause all sorts of problems.

4: has anyone done away with the 4 way PDWA brass body and used 2 brass straight connectors to create a front and rear brake circuit.

Thank you for any help Mick.

 

[Rut]

Mick,
If you decide to rebuild the shuttle it’s really just the 2 o rings. It’s probably best to take it off the car and do it on the bench so you can do a thorough inspection and cleaning. I’m pretty much in the stock camp as far as brakes are concerned and I would make sure the switch is working (electrically and seal wise) and reinstall it.
Rut

 

[Tybalt]

Thanks to the good folks at Buckeye Triumphs, here's a nice set of instructions for overhauling the brake master and the PDWA. The later PDWA with the O-rings are easier to rebuild, the TR250 and most (maybe all?) 1969 TR6 cars were fitted with a PDWA that used very small cupped seals like a wheel cylinder and I have no idea where one could source those.

https://static1.squarespace.com/static/5c6dec53b10f25d4edf0b3f7/t/5d019238b9af3e000109c65f/1560384058350/Overhauling+Master+Cylinder+%26+PDWA.pdf

 

[RGK]

Recently did it. I removed it with the MC, very simple job. Just ensure you get the correct seals for your shuttle, there are 2 I know of.

 

[Tybalt]

Recently did it. I removed it with the MC, very simple job. Just ensure you get the correct seals for your shuttle, there are 2 I know of.

Since you have a TR250 with the cup seals, where did you get those or did you switch to the later o-ring style piston? I had not looked until this morning but see that Moss now offers a couple of PDWA repair kits. One is for the o-ring style PDWA piston, the other more expensive kit has an o-ring style piston included but the *.pdf information sheet associated with both kits reference MGBs. Both kits include two pairs of o-rings in different sizes so if one were to go the Moss kit route it would critical to select the correct pair of o-rings to use. Since the OP has a 1975 it should be the AS568-008 EPDM o-ring and assuming this to be the case, a bag of 100 from McMaster-Carr would still cost much less than the o-ring kit from Moss. McMaster #1289N108

When we tried to source those cup seals several years for a friend's TR250, we had no luck. I offered up an o-ring one style but he elected to just make up some brake pipes that did away with the PDWA.

 

[mhossack]

Thank you all for your replies.

I have decide to go a little rogue.

I have read you can’t rely on the PDWA piston to move if you lose either your front or rear brake circuit, and it is basically only a switch with a warning light, as mine is leaking I think this is a weak point in a 50 year old braking system, so I have decided to keep the 2 separate brake circuits which I think is a good idea.

I have a very good hydraulic parts supplier near me, I could have gone for straight brake pipe unions, but I liked the 3 way brake pipe union better, because I could use the fixing hole to bolt them together and look more like the original not that bothers me, I will attach a photo.

Mick.

 

[mhossack]

The final solution for me, see attached photo, and all seems good.

I think the PDWA valve is a poor design, if you only have a leaking shuttle O ring you lose brake fluid through the switch, and eventually either your front or rear brakes, the switch at least should be hydraulic fluid tight. I appreciate you can get fluid leaks elsewhere without any other back up.

When I removed the PDWA valve I found I needed 1 x 3/8 UNF brass 3 way brake union and 1 x 7/16 UNF brass 3 way brake union and not the 2 x 3/8 UNF brass 3 way brake unions I first thought, posted before.

I thought my first solution looked a bit crude, I was going to either use self-tapping screws or nuts and bolts to hold it in place, but removing the PDWA valve I found it was held in place by a UNF bolt and a captive nut below, so this was a good time to redesign my solution and use the original Triumph fixing point.

Mick.